Out of those 2, it would be a hard choice. Having the latest and greatest is always good because you get the newest features, like Apple Pay and a bigger screen. But, 16 gigs isn't a lot, and 32 I'd definitely recommend. If you really want a bigger screen and Apple Pay, go with the 6, but if storage is more important then get the 5s.
My personal recommendation is, like many others, save for a 64 GB 6, and get the latest phone plus all of the newest features plus a lot of storage. I know it is a lot, but it would be worth it if you went this route.
